Since 2014, HIV care and treatment services among key populations including female sex workers (FSWs) have intensified in Tanzania. We sought to track the epidemic among FSWs in Dar es Salaam, Tanzania. We conducted a cross-sectional integrated bio-behavioral survey using respondent-driven sampling and a structured questionnaire. Blood was drawn for HIV testing. Modified Poisson regression was used to determine factors associated with HIV infection. We recruited 958 FSWs (median age 26 years) of whom 952 consented to HIV testing. The HIV prevalence was 15.3% (95%CI: 12.5-18.6). Factors associated with higher HIV prevalence included old age (25-34 years: aPR 2.38; 95%CI: 1.23, 4.60 and over 35 years: aPR = 6.08; 95%CI: 3.19, 11.58) and having experienced sexual violence in the past year (aPR = 1.94; 95%CI: 1.34, 2.82). Attaining higher education level was associated with lower HIV prevalence (aPR = 0.51; 95%CI: 0.36, 0.73 for primary school level and aPR = 0.20; 95%CI: 0.08, 0.46 for secondary school level and/or above). The HIV prevalence among FSWs in Dar es Salaam has decreased by half since 2013. Prevention strategies should target older FSWs, aim to educate young girls, and institute approaches to mitigate violence among FSWs.